The discharge into the environment of industrial wastewaters containing dissolved heavy metals is often prohibited for environmental, health and safety reasons. Wastewater contaminated with heavy metals could be toxic to aquatic life as well as unfit for human consumption.
U.S. Pat. No. 4,465,597 to Herman et al. discloses a method for removing dissolved heavy metals from an acidic industrial wastewater with a pH of between 2.5 and 5.0 by introducing a neutralizing agent adsorbed on the surface of a carrier which can be silica, sand, alumina, recycled metal oxides, recycled sludge formed in the method, or combinations thereof.
The Herman et al. method is limited to removing dissolved heavy metals from industrial wastewater having a pH between 2.5 and 5.0. This restriction results from the fact that certain heavy metals which precipitate at one pH level may redissolve upon further pH adjustments and, hence, recontaminate the resulting effluent since the solubility of various heavy metals varies with the pH of the solution in which they are dissolved.
Additionally, the prior art method uses a substantially instantaneous pH adjustment process, which is more likely to form amorphous insoluble calcium salts, as opposed to calcium salts in the monoclinic crystal form. The amorphous insoluble calcium salts tend to deposit on the surfaces of equipment, causing clogging and, hence, temporary shutdown of equipment operations for cleaning and maintenance.